From his first months in office,President Obama secretly ordered increasingly sophisticated attacks on the computer systems that run Irans main nuclear enrichment facilities,significantly expanding Americas first sustained use of cyberweapons,according to participants in the programme.

Obama decided to accelerate the attacks  begun in the Bush administration and code-named Olympic Games  even after an element of the programme accidentally became public in the summer of 2010 because of a programming error that allowed it to escape Irans Natanz plant and sent it around the world. Experts studying the worm,developed by the US and Israel,called it Stuxnet.

At a tense meeting in the White House Situation Room within days of the worms escape,Obama,Vice-President Joseph R. Biden Jr. and the director of the CIA at the time,Leon E. Panetta,considered if Americas most ambitious attempt against Iran nuclear efforts had been fatally compromised.

Should we shut this thing down? Obama asked,according to members of the presidents national security team. Told it was unclear how much the Iranians knew about the code,and offered evidence that it was still causing havoc,Obama decided the cyberattacks should proceed. In the following weeks,the Natanz plant was hit by a newer version of the worm,and then another.

The last of that series of attacks,a few weeks after Stuxnet was detected globally,temporarily took out nearly 1,000 of the 5,000 Iranian centrifuges. This account of the US and Israeli effort to undermine the Iranian nuclear programme is based on interviews over the past 18 months with current and former US,European and Israeli officials. None allowed their names to be used.

These officials gave differing assessments of how successful the sabotage programme was. The Obama administration estimates Irans nuclear ambitions were set back by 18 months to two years,but some experts are more skeptical. Whether Iran is still trying to build a nuclear weapon is in dispute.

The US government only recently acknowledged developing cyberweapons. There have been reports of one-time attacks against al-Qaeda. But Olympic Games appears to be the first time the US has repeatedly used cyberweapons to cripple another countrys infrastructure,achieving,with computer code,what otherwise required bombing or sending agents into a country. The code itself is 50 times as big as the typical worm,said Carey Nachenberg,a vice-president of Symantec,which dissected the code.

An analysis is now under way to decipher another cyberweapon called Flame recently attacked Iranian computers. But US officials say it was not part of Olympic Games and have declined to say if the US was behind it.

Obama,officials said,was aware that with every attack he was pushing US into new territory,much like with the first use of atomic weapons in the 40s,of intercontinental missiles in the 50s and drones in the past decade.